Vertical liquid phase crystal growth apparatus

ABSTRACT

A vertical liquid phase crystal growth apparatus includes (1) a container for housing a melt and either a single substrate or a plurality of substrates, (2) a furnace, and (3) a heat sink. The melt and the substrates are isolated from one another prior to the growth process by means of a cover plate which restricts the free volume surrounding the substrate(s). The container is heated to the desired temperature and the saturated melt containing the desired dopants and the substrate are allowed to come into contact by lifting the plate. Growth is then initiated from a convection-free melt by cooling the container and, thus, the substrate by means of a heat sink which establishes a thermal gradient along the vertical axis of the container.

ijiit gmgllll 1 1 Mar, 11A, 1972 [54] l/lElit'lllliCAlL LllQlUllD PHASE1 GIPUWTHI APPARATUS YS'lFAlL 211 Appl. No.: 119,929

3,491,720 1/1970 Harris ..1 113/495 3,524,426 8/1970 Ogle, Jr. et a1.....118/49 3,551,219 12/1970 Panish et al ....143/l7l 3,560,276 2/1971Panish et al... ....l48/171 3,565,702 2/1971 Nelson ..148/172 PrimaryExaminer-lv1orris lKaplan Attorney-R. J. Guenther and Edwin B. Cave [57] AHSTPACP A vertical liquid phase crystal growth apparatus includes(1) a container for housing a melt and either a single substrate or aplurality of substrates, (2) a furnace, and (3) a heat sink. The meltand the substrates are isolated from one another prior to the growthprocess by means of a cover plate which restricts the free volumesurrounding the substrate(s). The container is heated to the desiredtemperature and the saturated melt containing the desired dopants andthe substrate are allowed to come into contact by lifting the plate.Growth is then initiated from a convection-free melt by cooling thecontainer and, thus, the substrate by means of a heat. sink whichestablishes a thermal gradient along the vertical axis of the container.

5 Claims, 41 Drawing Figures 2 Sheets-Sheet 1 lNfl/ENTOR A. A. BERGH'ATTORNEY Patented March 14, 1972 2 Sheets-Sheet 2 m V//////////Z////AVERTICAL LIQUID PHASE CRYSTAL GROWTH APPARATUS BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to a verticalliquid phase crystal growth apparatus and more particularly, to avertical apparatus which incorporates a heat sink means to establish avertical temperature gradient during growth.

2. Description of the Prior Art Heretofore, crystal growth from theliquid phase has been accomplished with a multiplicity of verticalliquid phase apparatus. However, the quality of the regrown layers,especially in epitaxial growth has been adversely affected by thequality of the substrate material as well as by temperature gradients inthe melt. Surface degradation of the substrate most often occurs from(I) decomposition of the substrate followed by volatilization of anelement therefrom at elevated temperatures, (2) the transfer of volatilecompounds from the melt to the substrate and (3) the transfer of foreignparticles to the substrate surface from the melt.

An apparatus which restricts the free volume surrounding the substrateand which physically separates the substrate from the melt wouldalleviate the first two above-mentioned causes of surface degradation.Since most impurities have a lower density than the usual melts employedin liquid phase growth, small particles of contamination usually floatatop the melt. Apparatus has been developed heretofore in which the meltis skimmed" prior to contacting the substrate; however, the cleansurface required entails extreme care, and it would be much easier andsafer to construct an apparatus which prevents the substrate fromcontacting the liquid-gas interface, thereby eliminating the thirdabove-mentioned cause of surface degradation.

In addition to the above-noted difficulties, convective currents in themelt during regrowth result in (I) an irregular thickness of the regrownsurface, (2) a variable doping profile over the regrown layer and (3) adamaged interface between the substrate and the regrown layer. Ifcooling starts at the top of the melt or if regrowth is initiated at thetop, gravity causes the heavier solution to sink and form convectivecells. To avoid convection, a vertical temperature gradient must beestablished with a decreasing temperature toward the lowest part of thesystem. However, the apparatus should have a horizontal temperaturegradient of zero in order to obtain a uniform thickness of the regrownlayer.

SUMMARY OF THE INVENTION The present invention is directed to a verticalliquid phase crystal growth apparatus, which optimizes the growth by (l)maintaining the free volume surrounding the substrate at a minimum, (2)physically segregating the substrate from the melt prior to growth, (3)preventing melt-substrate contact at the liquid-gas interface and (4)providing a vertical thermal gradient with isothermal conditions in thehorizontal direction whereby the substrate is maintained at the lowesttemperature.

The vertical apparatus consists of a container for housing a melt and asubstrate. The base of the container has a compartment or well whichintimately accommodates the sides of the substrate. Isolating thesubstrate from the melt when the container is maintained at an initialor first temperature is a cover plate which covers the substrate.Attached to the cover plate is a rod which raises the cover plate fromthe substrate when the container is maintained at a second temperature,thereby leading to contact between the substrate and the melt. Coveringthe container is a lid which insures uniform heat dissipation from thecontainer and through which the rod slidcably passes. Contacting thebase of the container is a heat sink which insures a verticaltemperature gradient during growth.

In operation, a suitable substrate is placed in the container within thewell. The cover plate is placed over the substrate and a melt mixturecontaining the material to be grown and the required dopants is placedin the container atop and surrounding the cover plate. The lid isaffixed in place and the container is heated to a first temperature toform a solution from the melt mixture. Upon reaching a secondtemperature, the cover plate is raised so as to uncover the substrateand allow the saturated melt to come in contact with and cover thesubstrate. Crystal growth is then allowed to proceed by cooling thesubstrate, employing a heat sink means. A vertical temperature gradientis provided to insure deposition from a convection-free melt.

DESCRIPTION OF THE DRAWING The present invention will be more readilyunderstood by reference to the following drawing taken in conjunctionwith the detailed description, wherein:

FIG. 1 is a perspective view of the vertical liquid phase crystal growthapparatus of the invention;

FIG. 2 is a cross-sectional view of the growth container after beingloaded with the substrate and the melt mixture;

FIG. 3 is a cross-sectional view of the loaded container containedwithin the vertical furnace; and

FIG. 4 is a cross-sectional view of the apparatus during crystal growth.

DETAILED DESCRIPTION The present invention has been described largely interms of the epitaxial growth of P-type GaP on a substrate of P-typeGaP. However, it will be understood that such description is forpurposes of exposition and not for purposes of limitation. It will bereadily appreciated that the inventive concept described is equallyapplicable to nonepitaxial as well as epitaxial growth and to crystalgrowth of nonsemiconductor materials as well as semiconductor materials.Also the inventive concept described is applicable to many combinationsof substrate and melt whereby homojunctions and heterojunctions areformed. Regarding the epitaxial growth of semiconductor materials, thematerials may be selected from among group III(a)-V(a) compounds, groupllI(b)-VI(a) compounds or group IV elements of the Periodic Table of theElements as set forth in the Mendelyeev Periodic Table appearing on pageB2 in the 45th edition of the Handbook of Chemistry and Physics,published by the Chemical Rubber Company.

With reference now to FIG. 1, there is shown the vertical crystal growthapparatus of the present invention. Shown in the figure is a containeror boat 61, which can be fabricated from any inert material includingsuch materials as high purity graphite, alumina, quartz, boron nitride,or any inert ceramic material. It is to be understood that all of theabove-mentioned materials, including graphite, may or may not beemployed with a high purity graphite liner (not shown). It is also to beunderstood that although the container or boat 61 has been shown to berectangular in shape, the inventive concept need not be restrictedthereby and the container may be parallelepiped or cylindrical in shape.

The container 61 has a compartment or well 62, on its base 63, destinedfor accommodating a substrate 64 flush against the walls 66 of thecompartment 62. Covering the container 61 is a lid 67 which isconstructed of the above-mentioned inert materials and which serves toinsure uniform heat dissipation from the container 61.

A rod 68 made of the above-mentioned inert materials slidcably passesthrough an aperture 69' in the lid 67. Attached on one end 71 of the rod68 is a cover plate 72 which may be made of any of the above-mentionedmaterials. Preferably, the cover plate 72 should be constructed of thesubstrate material, e.g., GaP, to decrease the surface degradation ofthe substrate via loss of any volatile elements therefrom and to assurea saturated melt. It is to be noted at this point, that when thesubstrate 64 has been inserted into container 61, the top surface 73 ofthe substrate 64 is almost flush with the top surface of the base 63.Therefore, when the cover plate 72 covers the substrate 64 (FIG. 2) thefree volume 76 surrounding the substrate 64 is thereby maintained at aminimum. This minimization decreases the elemental loss due toevaporation from the substrate. The free volume maintained shouldideally be one which limits the loss due to evaporation to one monolayerof the surface of the substrate. The free volume is therefore dependentupon the substrate employed and the vapor pressures involved.

The container 61 is contained within a vertical furnace 74, supportedtherein by a pedestal 78. Incorporated within the furnace 74, as shownin FIGS. 1 and 3, is a heat exchanger 79, wherein gas maintained at lowtemperatures is introduced via inlet 81 through the heat exchanger 79and out through outlet 82. In this regard, it is to be noted that theheat exchanger has been described in terms of a cooling gas exchanger,but the in-,

ventive embodiment is not to be restricted thereby and any heat sinkmeans may be employed.

Although the invention has been described for illustrative purposes interms of one substrate and one compartment for retaining said substrate,it will be understood that the apparatus may include a plurality ofsubstrates accommodated within a plurality of compartments, all of whichare within a single container and covered by either one large coverplate or a plurality of cover plates.

Referring now to an exemplary technique, a suitable P-type GaP substratematerial grown by standard liquid encapsulated pulling techniques is cutto size whereupon it is lapped and cleaned in accordance withconventional techniques. Referring to FIGS. 1 and 2, the lapped andcleaned GaP substrate 64 is inserted into a high purity graphitecontainer 61 similar to that described in FIG. 1. This loading isaccomplished in an inert ambient such as nitrogen. The substrate 64 isplaced into well 62 and is thereupon covered by a cover plate 72 whichis fabricated of GaP and to which is attached a high purity graphite rod68.

A galliumGaPGa O -Zn melt mixture 75 is prepared by first weighing outhigh purity gallium, Zn and 03,0, obtained from commercial sources. Thehigh resistivity GaP material is cut and weighed out and then added tothe gallium, Zn and Ga O previously weighed out and the mixture 75 isadded to the container 61 atop of the cover plate 72. The amount of GaPemployed is such as to form a saturated gallium solution at the desiredtemperature. Typically, the composition meeting the saturationrequirement is in the range of 0.2 to 12 mole percent GaP, in thetemperature range of 800-1,200 C. v

After the melt mixture 75 has been inserted into the container 61, ahigh purity graphite lid 67 is placed over the container 61, rod 68passing through aperture 69 of the lid 67. The lid 67 covers container61 and insures uniform heat dissipation therefrom. Referring to FIG. 3,the container 61 is then placed within a standard vertical furnace 74upon pedestal 78. Thermal insulation 84 is packed around the pedestal 78thereby creating an isothermal center portion which accommodates thecontainer 61.

The furnace 74 is flushed with nitrogen admitted to the system throughsuitable inlet and outlet means (not shown). After flushing withnitrogen, hydrogen is allowed to flow into the furnace 74 and thecontainer 61 is heated to a first temperature above the melting point ofgallium thereby resulting in the formation of a melt 83. At this point,it should be noted that the substrate 64 is isolated or segregated fromthe melt 83 by means of the GaP cover plate 72 which prevents thetransfer of volatile compounds to the substrate 64 prior to deposition.Also, the free volume 76 surrounding the substrate 64 is maintainedideally at 0.01 mm. to restrict the loss due to volatilization toapproximately one monolayer of the surface of the Gal substrate 64.

The container 61 is heated to a second temperature in the range of1,050-] ,060 C., resulting in the formation of a saturated P-type GaPgallium melt 83 doped with zinc and oxygen. Referring to FIG. 4, afterthermal equilibrium is established between the substrate 64 and the melt83, the cover plate 72 is lifted by raising rod 68 which results in thecovering of the substrate 64 by the saturated melt 83. Due to theisothermal conditions, neither deposition on the substrate nordissolution of the substrate occurs at this time. Also, the substrate isprevented from surface contamination via contact with the gas-meltinterface.

A cooled inert gas such as nitrogen is flowed into inlet 81, throughheat exchanger 79 and out through outlet 82. This causes the temperatureof the pedestal 78, the base 63 and the substrate 64 contained withinthe container 61 to be lowered. A temperature gradient is establishedalong the vertical axis of the container 61, the base 63 and thesubstrate 64 wherein the base 63' and the substrate 64 are at the lowesttemperature. Crystal growth is thus initiated through the cooling of thesaturated melt 83.

The growth process is terminated by lowering the cover plate 72 over thesubstrate 64. In the alternative, the furnace 74 can be tilted, therebytipping off the melt 83 from the substrate 64. Typically, the GaPsubstrate 64 is in contact with the melt 83 from 4 to 50 minutes leadingto a grown crystal layer of from A to 3 mils.

What is claimed is:

1. A vertical liquid phase epitaxial growth apparatus, which comprises:

a. a container for housing a melt and a substrate;

b. a compartment within the base of said container for bolding saidsubstrate;

c. means for heating said container;

d. cover plate means closing off said compartment for isolating saidsubstrate from said melt when said container is maintained at a firsttemperature, said cover means comprising a portion of the bottom supportfor said melt;

e. means adapted to raise said cover means whereby said cover is removedfrom its isolating position and said substrate is contacted by the meltwhen the container is maintained at a second temperature; and

f. heat sink means contacting the base of said container forestablishing a vertical temperature gradient along the vertical axis ofsaid container.

2. The apparatus as defined in claim 1 wherein:

said compartment is adapted to intimately accommodate the sides of saidsubstrate.

3. The apparatus as defined in claim 1 wherein said heating means is avertical furnace.

4. The apparatus as defined in claim 1 which further comprises a lid forcovering said container for insuring a uniform heat dissipation fromsaid container and wherein said raising means comprises a rod attachedto said cover plate and slideably fitted through said lid.

5. The apparatus as defined in claim 3 wherein said heat sink is a heatexchanger maintained within said vertical furnace.

2. The apparatus as defined in claim 1 wherein: said compartment isadapted to intimately accommodate the sides of said substrate.
 3. Theapparatus as defined in claim 1 wherein said heating means is a verticalfurnace.
 4. The apparatus as defined in claim 1 which further comprisesa lid for covering said container for insuring a uniform heatdissipation from said container and wherein said raising means comprisesa rod attached to said cover plate and slideably fitted through saidlid.
 5. The apparatus as defined in claim 3 wherein said heat sink is aheat exchanger maintained within said vertical furnace.